Large area solid state x-ray detectors are currently being developed in the x-ray art. Such a detector typically comprises a scintillating layer in contact with an array of photodiodes, each with an associated FET switch. The photodiodes are initially charged by connecting them to a known stable voltage through the switches. Subsequently, the photodiodes are isolated by turning the FETs off. Upon exposure to x-rays, the scintillator produces light which discharges each photodiode in proportion to the x-ray exposure at the position of the diode. The diodes are then recharged by again connecting them to the known stable voltage. The charge used to restore the diode to its initial voltage is measured by a sensing circuit, and the value is digitized and stored.
In addition to an x-ray induced signal, the signal of each pixel includes an offset which is independent of x-ray exposure. This offset has several sources including leakage current in the photodiodes and charge retention in the FET switches. At low signal levels, such as found in fluoroscopic imaging, the magnitude of the offset can be larger than the x-ray signal. Furthermore, the offset is not uniform, but varies from pixel to pixel in such a manner that it interferes with the visualization of information in the x-ray image. This pixel-dependent offset must be subtracted from the x-ray exposed image to produce a corrected image before viewing.
One means of isolating the offset from the x-ray induced signal is to acquire an image when the detector is not exposed to x-rays, i.e., a dark image, with the same timing used for acquisition of the x-ray image, in order for the signals in the dark image to match the offset signals in the x-ray image. Because there is noise associated with the offset signals, a single dark image subtracted from an x-ray image would introduce additional noise into the corrected image. To reduce the amount of this noise, several dark images could be averaged together to obtain a low-noise offset image. However, the offset signals may drift with time, temperature, and other external factors. Therefore, the offset image must be updated periodically.
It would be desirable then to have a means for periodically updating the offset image.